It is often beneficial to monitor the condition of filter elements. One of the most common ways to monitor the condition of filter elements is to measure a pressure drop across a filter vessel using a differential pressure gauge. A measurable difference in pressure occurs over time because, in a clean filter, a fluid being filtered flows freely without restriction, and in a filter filled with dirt and debris, the fluid flow is restricted. Accordingly, a fluid flowing through a clogged filter will have a higher pressure before entering the filter and a lower pressure after exiting the filter.
Many types of differential pressure gauges are available. For example, a piston-type gauge can be used to measure pressure drop. A piston-type gauge typically includes a piston that is tightly fit into a cylinder, with a spring attached to one end of the piston. A fluid having a higher pressure is directed by tubing to one end of the piston while a fluid having a lower pressure is directed by tubing to an opposite end of the piston. The spring is configured to oppose motion of the piston in a direction from the higher pressure source to the lower pressure source. The pressure from each respective fluid source exerts a force on each respective end of the piston. When the difference in pressure between the two ends of the pistons is zero, the spring does not compress or extend and the piston is at rest. However, as the difference in pressure becomes greater, the piston is displaced, compressing or extending the spring until the forces on each side of the piston are equal. The position of the piston indicates the difference in pressure between the two ends of the piston.
The position of the piston may be determined in various ways. For example, the cylinder may be clear, allowing a user to actually see the altered position of the piston. Often this clear cylinder is in the form of a glass tube. There are, however, many disadvantages to housing the piston in a glass tube. One common problem is that the glass tube and the piston must each be formed from materials having a similar coefficient of thermal expansion for the piston to retain its close fitting relationship within the glass tube, especially when the piston-type gauge experiences a wide range of temperatures during use. Unfortunately, those materials available for forming the piston to have a coefficient of thermal expansion similar to that of glass are typically expensive metallic alloys, such as Invar 36, that greatly increases the cost of the piston-type gauge. These high cost metallic alloys are also problematic as they often are difficult to machine without causing serious burring in the metallic alloy, which in turn affects the quality of the seal between the piston and the glass tube.
Another problem associated with the use of a glass tube as a housing for the piston is that the strength of a glass tube decreases as a selected diameter of the glass tube is increased, prohibiting the piston-type gauge from utilizing a larger diameter glass tube if high internal pressures act on the glass tube. This occurs because the wall tension found in a cylindrical chamber is directly proportional to both the diameter of the cylindrical chamber and the internal fluid pressure within the cylindrical chamber, meaning that larger diameter glass tubes experience greater wall tensions than smaller diameter glass tubes exposed to the same internal pressure. Accordingly, larger diameter glass tubes fail due to wall tension at lower internal pressures than do smaller diameter glass tubes.
When the glass tube is used to house the piston, the diameter of the glass tube also limits the maximum diameter of the piston to be no greater than the inner diameter of the glass tube. This in turn limits the accuracy of the differential pressure reading. This occurs because the piston is manufactured to have certain tolerances in regards to the manufactured dimensions of the piston. Smaller dimensioned parts introduce a greater percentage of error than do larger dimensioned parts. Because the accuracy of a piston-type gauge is largely determined by an accurate measurement of the surface area of the piston end contacting the pressurized fluid, use of larger diameter pistons with less associated error is preferable.
Alternatively, other piston-type gauges utilize a magnetic sensor to show the position of the piston. However, disadvantages of using a piston displayed through a magnetic sensor include expensive materials and manufacturing processes, increased errors due to damage or interference from the presence of contaminants of a magnetic nature between the piston and the cylinder, and a general decrease in the accuracy of the pressure difference readings due to unnecessary components.
It would be desirable to produce a differential pressure gauge including a remote indicator located remotely from a sensing area of the piston that minimizes errors, maximizes reliability, does not attract contaminants of a magnetic nature, and is inexpensive to make.